What is generally used to produce melamine from urea?

A method for purifying melamine containing impurities such as urea melamine, oxotriazine, melaminol, and millerine, which comprises the following steps: bringing a melamine melt into contact with a catalyst bed at a temperature of 340°C–410°C and a pressure of 8–17 MPa, thereby obtaining melamine with a purity of at least 99%. 　

We use the atmospheric pressure method, with one-step crystallization and quenching to produce the final product.

Process Flow The main raw materials for melamine production are urea and liquid ammonia. Ammonia gas is used as the carrier gas in the reactor; after preheating, it is fed into the bottom of the reactor to fluidize the catalyst there. Granular urea is added intermittently from a urea tank, and compressed air is used to push the urea into the reactor, where it undergoes high-temperature decomposition under the action of the catalyst to form melamine. Since the reaction is endothermic, the heat required for it is provided by the molten salt circulation system. The high-temperature gas exiting the fluidized bed reactor passes through dry catchers first, where it cools naturally; melamine, the product of this reaction, along with the polymers formed as by-products, cool down and precipitate as solids, accumulating within these dry catchers. After leaving the dry catchers, the exhaust gas enters four purification towers in sequence. Within these towers, the exhaust gas comes into countercurrent contact with water, resulting in the formation of ammonium carbamate. The ammonia gas emerging from the fourth tower is dried and compressed before being reused. The crude melamine is sent to a refining process where it is heated and dissolved using water as the solvent. Activated carbon and other additives are added to the dissolution tank; thereafter, filtration takes place to remove impurities that are insoluble in water. The filtrate then enters a crystallization tank where it cools and crystallizes. The slurry is passed through a centrifuge, and the filtrate is returned to the dissolution tank for reuse. The wet melamine is dried in a dryer to produce the final product. The specific production equipment is shown in Table 3-9, while the production process is illustrated in Figure 1-2. The chemical reaction equation is as follows: 6(NH2)2CO = C3N6H6 (melamine) + 3CO2 + 6NH3

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